Bright light produced by laser devices is today utilized in a multitude of different processes. Such processes include e.g. material marking, surface treatment, cutting and welding. High power lasers are applicable e.g. for handling of metal materials using the above-mentioned processes. Lasers are also widely applied for medical purposes and in various optical measurements.
Since diode lasers based on semiconductors are developing, increasing the optical power obtained thereby and improving the beam quality, applications using compact semiconductor lasers have been strongly expanded for the above-mentioned application areas, which have traditionally been using larger size gas and crystal lasers. Semiconductor laser devices achieve typically a high optical total power by combining radiation emitted by a plurality of single laser emitters.
A generic purpose for a semiconductor laser device with a large power density is to produce a sufficiently powerful and bright beam of light that can be focused to the object in a desired manner. The brightness of the light source is determined as the light power radiated from an emitting surface area to a certain solid angle. The brightness of a single light source cannot be increased by passive optical elements.
As for semiconductor lasers, the light power of a single semiconductor emitter is fairly limited, wherein in order to obtain a sufficient total light power a large number of single emitters are required. To increase the light power, so-called laser bars are used which are composed of a plurality of single emitters arranged side by side, multiple of which laser bars can be further combined one above the other to constitute so-called laser towers. Nevertheless, as the number of the emitters in a light source is thereby increasing, it is inevitable that also the size of the light source increases. In order to increase the power density of the radiation emitted by the light source the beams originating from different locations of the light source should be combined together in a suitable manner. This combining of beams is generally referred to as multiplexing. Different multiplexing methods used in connection with semiconductor lasers include wavelength, polarization and spatial multiplexing.
In wavelength multiplexing the beams of two or a plurality of (n pieces) different wavelengths are combined to constitute one beam that uses beam combiners having suitable wavelength dependencies. Such beam combiners that are well known as such in the field of optics are e.g. dichroic mirrors, known examples of which include e.g. so-called hot/cold mirrors. Upon using beam combiners the diameter of the combined beam can be arranged to correspond substantially to the diameter of each beam to be combined and having a different wavelength. Hence, the power density of the combined beam is increased in a free-of-losses situation to n-fold. Since the solid angle of light can be kept original, also the brightness of the combined beam grows in the same relation as the power density.
In polarization multiplexing two beams having the same wavelength is combined to one beam using a polarization beam combiner. If necessary, the polarization level of the other partial beam can be turned e.g. 90° with a λ/2-plate. Since the diameter of the combined beam is substantially the same as the diameters of the partial beams that are to be combined, the power density grows in this case practically almost to double. Because the solid angle of light does not change, also the brightness grows almost to the double.
In spatial multiplexing the beams originated from different light sources are collected to the same location in space. The purpose of spatial multiplexing is to maintain the brightness of the original light source as well as possible. The power density can be increased, but since the solid angle of light grows in the same relation, the brightness cannot be increased with mere spatial multiplexing.
In practice it is very difficult to design and implement a structure that would combine a plurality of semiconductor laser beams in an efficient manner and simultaneously use wavelength, polarization and spatial multiplexing. This is due e.g. to the astigmatic quality of the emission of semiconductor lasers, and the width of prior art laser bars.
Typically the height of the light-emitting surface of single emitters used in prior art broad laser bars (later referred to as direction y) is less than 1 μm and in this rapidly diverging direction (fast axle, FA) the beam that originates from the emitter is spreading in a Gaussian manner at an angle of 30 to 40° (FWHM). The light-emitting surface width of individual emitters (later referred to as direction x) is typically of the order 100 μm and in this slowly diverging direction (slow axis, SA) the outgoing beam is diverging at an angle that is less than 10° (FWHM). Inactive, “empty” space that does not emit light is always present between adjacent emitters that have been combined to a laser bar. A laser bar that is typically of the order 10 mm and contains 20 to 40 adjacent emitters produces 20 to 50 W of continuous light power. Proportioned to the length of the laserbar, the light power is then 2 to 5 W/mm. In pulsed use the momentary corresponding power can exceed 10 W/mm.
U.S. Pat. No. 5,825,551 discloses a simple spatial multiplexing method based on two plane mirrors. In the solution, the slow axis of the emitters is directed diagonally between two glass sheets, where the beams of light remain trapped and where they based on reflections travel towards the exit end of the glass sheets. The beams exit from a mirror gap between the glass sheets being narrower than originally. The structure is simple, but a part of the light power is lost as reflection losses. Furthermore, it is difficult to implement different multiplexing methods simultaneously using the referred method, wherein the maximum power density obtained is limited.
Several patents suggest using different types of waveguides for combining beams of laser bars (U.S. Pat. No. 4,820,010) or emitters (U.S. Pat. No. 6,312,166). Solutions based on waveguides remove non-emitted empty space between emitters and/or laser bars and they can be used for transferring light further to optical fibers. However, in practice waveguides often reduce the brightness of the light source in a significant manner. This is due to the fact that a high fill factor cannot be achieved at the exit end of the waveguides. The waveguides must have a sufficiently large light coupling end in order to collect all the light power, but often a part of the coupling surface is inefficient wherein the total brightness deteriorates. In addition, the accuracy of mounting can be a critical factor to the beam quality. Polarization and wavelength multiplexing cannot be directly connected to the use of the waveguides either.
To maintain the laser brightness in the slow axis direction is perhaps the most common problem in combining bright diode laser beams, for which problem solutions are disclosed in a plurality of patent publications. The problem can be solved simply in a manner that emitters are processed on a laser wafer so that they are located sufficiently far away from each other. Thus, in front of individual emitters then slow axis collimation optics with sufficiently large size relative to the size of each emitter (U.S. Pat. No. 5,793,783) can be placed. Nevertheless, such solutions are not advantageous in practice, as the exploitation level of the laser wafer will be poor because most of the wafer is composed of empty space between the emitters.
A plurality of solutions tend to narrow the light field in the direction of the slow axis by using optical methods and at the same time to possibly remove the empty space between the emitters. Generally it can be stated that the narrowing of the slow axis takes place by growing the height of the picture of the light source, which will inevitably involve that the physical distance between the laser bars will grow when combined by piling them one above the other. In addition, the physical space required by the narrowing and piling optics makes the combining of different multiplexing methods difficult. The main characteristics of known narrowing optics will be described in more detail in the following.
In U.S. Pat. No. 5,784,203 the narrowing of the slow axis is carried out using lens optics and parallel deflection utilizing a tower piled of glass sheets. The planes of the glass sheets of the tower are parallel, but the glass sheets have been rotated suitably relative to each other in a manner that the structure resembles a fan. A beam of light coming from a broad laser bar is divided in the direction of the slow axis to partial beams that are deflected in the direction of the fast axis, wherein the light that has originated from different emitters hits different layers in the glass sheet tower. Parallel shifts take place in the glass sheets that have been piled to form a fan structure, wherein beams originating from different emitters are arranged one above another in the direction of the slow axis. The light propagates in the glass sheet tower by means of total reflections, wherein medium of a lower reflection factor than the sheet itself is needed between the sheets and/or the sheet surfaces. Due to the diverging of the slow axis the light intended for different glass sheets is partly mixed and this part is lost. In this method a plurality of cylindrical lenses, ball lenses and glass lenses are needed, which adds to the size of the structure and makes simultaneous utilization of different multiplexing methods difficult. Narrowing and piling can also be done using two towers piled of glass sheets (U.S. Pat. No. 5,805,784 and U.S. Pat. No. 5,986,794). In this solution as well the laser bars remain far from each other and the method is therefore best suitable for fiber based coupling of light power originating from single laser bars.
A multitude of different narrowing and piling structures based on micromirrors have been introduced. For example, two rows of offset mirrors (U.S. Pat. No. 5,887,096) or a deflecting mirror system (U.S. Pat. No. 5,808,323) manufactured of semiconductors by semiconductor technique can be used for the purpose. U.S. Pat. No. 5,592,333 discloses narrowing optics which is based on monolithic mirror technique and which also removes the empty space between the emitters. The beam that has originated from each emitter or emitter group is reflected from both edges of a “V” shaped groove, wherein the direction of propagation changes 90° and the partial beams are piled one upon the other in a new manner. This results in splitting of the original long light beam and the formation of the split partial beams on top of each other in the direction of the fast axis, wherein these beams can be collimated with one lens. The structure of the device according to the method is, however, of “L” shape, which requires a large space. As a result of this, combining the beams of a plurality of laser bars is not very advantageous. Frequently, also problems related to the manufacturing technique of micromirrors are limiting factors. In optics based on mirror reflection, which the solutions above also represent, effect losses caused by absorption take place and the mirrors must possibly be cooled.
Narrowing and compressing the slow axis can be implemented in a manner corresponding to the step mirrors also by using glass bars of a suitable form (U.S. Pat. No. 5,877,898). The entrance surface of each glass bar is perpendicular to the incoming direction of light and the exit surface is 45° relative to the incoming direction of light. Due to the total reflection caused by an oblique exit surface the direction of light changes 90°. Solutions based on the total reflection show only minor power losses. However, small mirror structures integrated of glass bars are difficult to manufacture, and the laser bars cannot be brought close to each other in this solution either.
One practical solution for narrowing and collimating the slow axis is based on deflection prisms (U.S. Pat. No. 5,808,803). The beams of the emitter or emitter group are deflected in the direction of the fast axis by using prisms, wherein these beams that are narrower than the original laser bar can be collimated more accurately with their own cylindrical lenses. The number of the necessary collimation lenses depends on how many parts the original laser bar will be split into. The structure is simple, but the laser bars cannot be brought close to each other, because the deflection must be carried out in the direction of the fast axis. If the object is to collimate 20 emitters of laser bar separately, a 20-piece prism structure would be needed further with a 20-story collimation lens tower. Also the total brightness will be lowered which is unnecessary since the beams leaving from different emitters are not placed on top of each other in the direction of the slow axis.
The empty space between the emitters can be removed also before combining the beams, as has been done in U.S. Pat. No. 6,240,116 based on the step mirror structure.
As a summary of the above-described prior art solutions it can be stated that they tend to resolve the optical problems which are related to controlling the slow axis of the emitters and which are largely caused by properties of presently used broad laser bars. The above-described solutions facilitate the handling of the slow axis at the cost of more complicated structures and increased physical size of the technical solutions, which, in turn, makes the simultaneous usage of different multiplexing methods difficult and thereby limits the brightness and power density.